The conventional rotary throttle valve-type carburetor is designed so that turning of the throttle valve causes a needle to be moved up and down to adjust the extent to which a fuel nozzle is open. In low temperatures when the engine is cold, frictional resistance in the engine is high. Therefore, the engine is hard to start, and even if the engine is started its idle operation is unstable.
As shown in FIG. 51, a conventional rotary throttle valve-type carburetor has a carburetor body 5 provided with a cylindrical valve chamber 6 perpendicular to an air intake passage (extending vertically relative to the paper surface) a throttle valve 1 having a throttle hole 2 is rotatably and vertically movably fitted in the valve chamber 6. A valve shaft 1a of the throttle valve 1 extends through a lid plate 21 for closing the valve chamber 6, and a throttle valve lever 22 is connected to the upper end of the valve shaft 1a. A swivel 23 for connecting a remote control cable is rotatably supported on one end of the throttle valve lever 22, whereas a cam portion 22a is provided on the other end of the throttle valve lever 22. A peripheral cam groove with a depth that becomes gradually shallower in a direction of rotation corresponding to an increased throttle valve opening is provided in the lower surface of the cam portion 22a and a follower supported on the lid plate 21 is engaged with the cam groove to thereby constitute a cam mechanism.
Fuel is taken into a fuel nozzle of a fuel supply pipe 4 projecting toward the throttle hole 2 via a check valve and a fuel jet. In FIG. 51, the throttle valve 1 is in a fully open position, and the throttle hole 2 and the air intake passage are substantially registered or coincident in an axial direction. A needle 3 projecting downward from the throttle valve 1 is inserted into the fuel supply pipe 4.
In operation, to increase the speed and/or power of the engine, the throttle valve lever 22 is turned or rotated in an accelerating direction against the force of a spring to increase the extent to which the throttle hole 2 is open relative to the air intake passage. At the same time, the needle 3 is moved up by the aforementioned cam mechanism to increase the extent to which the fuel nozzle is open.
A start shaft 32 is fitted into a guide tube 53 formed integral with the lid plate 21, and when the start shaft 32 is turned by means of a start lever 31, a cam surface 52 formed on the end portion of the start shaft 32 lifts up the throttle lever 22 so as to increase the quantity of fuel. A pin 51 on the guide tube 53 is engaged with an annular groove of the start shaft 32 to retain the start shaft 32 in the guide tube 53.
In a small engine for a work tool provided with a centrifugal clutch and the aforementioned rotary throttle valve-type carburetor, when the airflow through the carburetor is increased sufficiently over the calibrated air flow for idle engine operation (thereby increasing the engine rpm at idle), the centrifugal clutch can become connected so that a tool driven by the engine is actuated, which may be undesirable. Accordingly, the airflow when the engine is started has to be set so that the speed (rpm) of the engine is slightly faster than the calibrated idle setting, but not so high as to engage the clutch.
However, after the break-in period of the engine, the set idle speed becomes faster than the value set after assembly at the factory. At this time, when the idling speed is adjusted to a proper value the increase in airflow at the start of the engine as adjusted by the start fuel increasing mechanism, can place the speed of the engine out of its desired range.